1. Field of the Invention
This invention is concerned with rapid setting high bond strength adhesives for bonding metals and plastics together and for bonding either or both to tooth enamel. Applications exist in dentistry, cosmetics and in a plurality of industrial fields as more fully developed hereinafter.
2. Description of the Prior Art
Adhesives heretofore utilized for bonding stainless steel or plastic orthodontic braces or brackets to tooth enamel have generally been of three types--i.e.,
(1) a blend of aromatic diacrylates with aliphatic diacrylates cured with a conventional catalyst--tertiary amine accelerator system,
(2) a blend of methylmethacrylate monomer with polymethylmethacrylate polymer using a peroxide or trialkyl borane curing catalyst,
(3) an aromatic diacrylate system containing conventional ultraviolet sensitive accelerators, cured with an ultraviolet gun.
All of these adhesives show poor water resistance, poor peel resistance and poor retention to tooth enamel in orthodontic service. All of these adhesives show poor adhesion also to stainless steel--long used for orthodontic brackets--and to polycarbonate plastics, today the preferred bracket material of many orthodontists for aesthetic reasons. Frequently as many as 50% of the brackets bonded with these adhesives are lost within 30 days of application. In the case of polycarbonate brackets, moreover, sanding does not materially improve adhesion--probably because they contain mold releas additives, e.g., silicone oil. In the case of stainless steel brackets it is necessary to perforate the base with multiple holes to improve mechanical as well as adhesive retention with any of the common adhesives described above.
The adhesives referred to--and particularly those including methyl methacrylate monomer as one constituent--have found cosmetic use in products for mending cracks in finger and toenails, lengthening the nails and the like. In these uses as in dental uses, however, products containing significant amounts of methyl methacrylate monomer have the disadvantages of unpleasant odor and of high evaporability (due to the low boiling point of the monomer), the latter attended by poor reproducibility of the composition and by a tendency of the cured composition to crack due to evaporation of monomer even after initial cure. In addition, methyl methacrylate monomer has an irritant effect on many individuals, producing dermatitis and other allergic reactions. In some cases, individuals treated with methyl methacrylate monomer-containing compositions have exhibited more severe toxic reactions.
In another dental application, polycarbonate glass-filler reinforced dental crown forms intended to serve as relatively long term (five to ten year) expectancy crowns over a crown preparation on a tooth have heretofore been seated with conventional dental cements of the phosphate type or with polycarboxylate acid cements prepared from polymethacrylic acid and zinc oxide. These cements, however, do not bond strongly to polycarbonate. Moreover, they tend to deteriorate in oral fluids with the result that the crown form often is not retained for its full life expectancy.
There is also a need for adhesives of high strength which will set in about 90 seconds for a plurality of industrial applications involving bonding of metals to plastics, bonding of different plastics to one another, patching of glass, etc. Examples of this need are in various electronic devices where plastic must be joined to itself or to metal, in tubing, piping and electrical insulation where polycarbonate, Teflon, polyethylene, polypropylene and similar plastics must be bonded to one another or to metals, in the assembly of outdoor signs, etc. For example, adhesives now used for joining plastic piping for irrigation and like uses often require in the order of two hours setting time in the field. Few adhesives having both the requisite strength and set time are now known--and those that are available for industrial uses are often too toxic for dental or cosmetic uses.